Paper suitable for capacitors and process of making the same



Patented Mar. 22, 1932 PATENT OFFICE AMOS L. AILEN', OI"

PITTSFIELD, MASSACHUSETTS ASBIGNOB TO GENERAL ELECTRIC COMPANY, ACORPORATION OF NEW YORK PAPER SUITABLE FOR CAPACITORS AND PROCESS OFMAKING- IHE sum nojprawm This invention relates broadly to a new andimproved process of making paper from kraft pulp or sulphate wood pulp,and to the new product derived from such process.

A principal object of this invention is the production of a kraft paperwhich is materially less than one-half mil (0.0005 inch) in thickness.

- Another object of this invention is the provision of a process for theproduction of such thin paper.

Still another object of this invention is to produce a condenserdielectric of paper which is very much cheaper than those at present inuse, and which due to its nature, imparts useful properties tothecondenser in which it is used.

Other and further objects of this invention will be more apparent as thedescription proceeds.

The importance of condensers in the electrical art is constantlyincreasing, and the.

need for cheaper as well as more efficient condensers is thereforeapparent. One means of producing more eflicient condensers is in theemployment of the most suitable dielectric material. Paper has been usedin the art as a dielectric material because of its dielectric propertiesand because it is comparatively cheap. However, in order to produce acompact and efiicient condenser very thin paper has been necessary. Itis known that the thinner the paper used as a dielectric the greater thecapacity of the condenser will be. For example, a decrease in thicknessof the paper from, say, 0.0005 inch to, say, 0.0004 inch, which is a20%decrease in thickness, will produce practically the same percentagemcrease in capacity of the condenser, assuming the same area ofelectrodes.-

In order to obtain very thin paper it has been necessary heretofore touse linen stock or pulp, which can withstand the severe heating whichhas been relied upon in the past to produce paper of the thinnessnecessary for a practical condenser dielectric. Paper suitable as aneflicient dielectric condenser material should be materially less than0.0005 inch in thickness. Wood sulphate 'pulp, which is ordinarily knownin the art Application filed January 25, 1930. Serial No. name.

as kraft pulp, is materially less in cost than l1nen pulp, but as far asI am aware, no one has been able prior to the present invention toproduce a paper from kraft pulp which is materially less than 0.0005inch in thickness. In fact, it has been common thought up to the presentinvention that such thin paper cannot be manufactured from kraft pulp byreason of the nature of the kraft pulp. The common belief has been thatthe kraft pulp could not withstand the length of the beat ng operat on,or the severeness of the beatlng operatlon for the length of time whichwould be necessary to produce such a thin paper, but would be so cut andshortened as to be unsultable for the desired purpose. Kraft pulp hastherefore, as far as I am aware, never been beaten in excess of about 12hours in making thin aper therefrom.

I have discovered that y properly controlling the conditions under whichkraft pulp is beaten I can produce ultimately a kraft paper which ismaterially less than 0.0005 IDCl'l in thickness. This I have discoveredI can do by controlling the beating action so that the severity of thebeating is increased as the pulp hydrates. In other words, by beginningwith a mild beating action and allowing the pulp to hydratesufliciently, then increasing the severity of the beating action as thepulp is able to withstand it, I avoid cutting the fibres of the kraftpulp and can carry the beating action on for a period conslderably inexcess of that heretofore thought possible. and produce a pulp whereinthe individual fibers are relatively fine and frayed out, and aresuitable for making an extremely thin paper.

In order that my invention may be more fully understood and mayconveniently be practiced by those skilled in the art to which itpertains, I shall describe the process I employ more in detail, andpoint out the salient features of the invention in the appended claims.

In carrying my invention into practice I proceed as follows:

A suitable quantity of kraft pulp is washed for about six hours toremove any chemicals which may be left therein at the J pulp The pulp isthen beaten with a suitable amount of water for about thirtysix to fortyhours at room temperature. The

beating apparatus commonly used .in paper maklng may be used for thispurpose. Durin the beating the pul heats up, due to frlctional heatdevelope to about 150 to 160 F. The beatingoperation according to theprocess of my inventionis carried on as follows:

After the stock has been placed in. the beater, the beater roll isadjusted so that the knives are just barely touching the bed plate. Thiscan be ascertained conveniently by taking a stick and after placing itagainst the bed late, lowering the beater roll (until a brus g noise ofthe roll against the bed plate is heard when the ear is placed againstthe stick. Having so adjusted the beater roll, the heater is run for adefinite interval, say, about four hours which is materially longer thanhas heretofore been the practice in heating kraft pulp. During this timethe stock begins to hydrate and becomes finer and finer. At the end ofthis period the roll is adjusted a little closer to the bed plate, say,about 0.001 inch closer, and the beating-is continued for anotherinterval of time slightly longer than the first interval, say, aboutfive hours. The stock-thus hydrates further. At the end of this intervalthe beater roll is again adjusted slightly toward the bed plate,

' for example, another 0.001 inch, and the beating continued.

This operation of successive adjustments and beating operations iscontinued, each time slightly increasing -the interval betweenadjustments, until the stock has been beaten about 36 hours. It will beseen that these operations permit the stock to gradually be- -'come moreand more hydrated, and as the stock thus becomes more and more hydrated,

; and can better withstand the beating action,

the beater roll is successively adjusted closer to the bed plate andsuccessively imparts more of its weight to the'p'ulp.

The degree of hydration can be observed by suitable test as the beatingproceeds, for example, by employing what is known in the art as theFreeness test, and also by examin ing samples at various intervals underthe microscope. When the Freeness test shows the proper value and themicroscopic examination of a sample shows the fibers of pulp to berelatively fine and frayed out, the heating operation may be consideredproperly at an end.

At the end of the beating operation the process assumes the steps whichare well known in the paper making art; that is, the

beaten pulp is taken to a beater chest whence it goes to a Jordan beateror refiner, from which it goes to the machine chest. From the latter thepulp is taken to a rifile box where the foreign particles such as iron,sand,

etc., are removed. The nextstep is to send the material to the papermachineand from the paper machine to the super calenders.

If the above. rocess is'carried out, 'con- 0.0004 inch in thickness. Incertain cases I have obtained aper of 0.00034 inch in thickness. The prouct obtained is extremely fine and has the pores thereof closed so thata sheet 0.0004 inch in thickness when held up to the light is veryuniform in texture or formation and is practically non-porous to thenaked eye. In contra-distinction to this, a sheet of linen paper of thesame thickness, when held up to the light, is not uniform in texture orformation to the naked eye.

Such extremely thin paper is suitable in many arts but is particularlyuseful in the electrical art as a dielectric material. While itpossesses the desirable characteristics of linen paper in thisconnection its cost is slightly more than'one-half that of linen.

paper of the same'thickness. It also produces a compact and efiicientcondenser.

The unbleached kraft paper is light brown in color but when bleached iswhite and possessesa beautiful gloss or sheen. Obviously, both thebleached and unbleached kraft paper of my invention are capable ofvaried organic esters, such as tricresyl phosphate, 1

which have been heattreated so that certain pyrogenic compounds arecontained therein,

or organic esters such as tricresyl phosphate, I

which have been heat treated in the presence of waxy materials all ofwhich is more fully v set forth in the co ending applications of FrankM. Clark, erial Nos. 425,357 and 425,359, filed Feb. 1, 1930, andassigned to the same assignee as the present invention. a

While in accordance with the present process I have been able to producethin kraft paper of the order of 0.0004 inch in thickness, it isentirely within the scope of my invention to produce even thinner paperthan this by simply controlling the beatin operation in the mannerdescrlbed above ut taking care to properly hydrate the pulp as it isbeaten.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is 1. A process for producing a thin kraft paper having athickness materially less than 0.0005 inch which comprises controllingthe beating action on the pulp by prolonging the intervals of timebetween successive adjustments of the beater roll with respect to thebed plate so that the pulp can hydrate sufficiently to withstand thenecessary beating action, and increasing the severity of beating at eachadjustment so as to produce ultimately fibers which will make a sheet ofthe thickness specified when subjected to paper making steps.

2. In a process for producing a thin kraft paper having a thicknessmaterially less than 0.0005 inch, the steps which comprise controllingthe beating action as follows: adj usting the beater roll so that itbarely touches the bed plate of the beater, beating the pulp for aninterval of time materially in excess of two hours with the beater rollin this position, successively decreasing by relatively small incrementsthe distance between the beater roll and the bed plate at intervalsapproximately the same as the first interval until the pulp has beenbeaten sufficiently to produce ultimately such a thin paper.

3. A process for producing thin kraft paper having a thicknessmaterially less than 0.0005 inch, which comprises controlling thebeating action on the pulp by successively increasing the severity ofthe beating action as the pulp hydrates suificiently to withstand suchbeating, and beating the pulp for a period of time suificient to producefibers which will "make a sheet of the thickness specified whensubjected to paper making steps.

4;. A process for producing a thin kraft paper having a thicknessmaterially less than 0.0005 inch which comprises washing the pulp as itis received from the mill, subjecting the washed pulp in the presence ofa. suitable quantity of water to a beating action for about 36 to 40hours as follows: adjustingthe beater roll so that it barely touches thebed plate of the beater, beating the pulp for about four hours with thebeater roll in this position, successively decreasing by relativelysmall increments the distance between the beater roll and the bed plateat intervals approximately the same as or slightly greater than thefirst interval until the pulp has been beaten for the requisite time,then subjecting the beaten pulp to the usual paper making steps.

5. A process for producing thin kraft paper having a thicknessmaterially less than 0.0005 inch, which comprises controlling thebeating action on the pulp by successively increasing the severity ofsuch beating action as the pulp hydrates sufficiently to withstand suchbeating, and beating the pulp by such controlled action until theindividual fibers are relatively fine and frayed out.

6. A thin kraft paper having a thickness materially less than 0.0005inch.

7. A thin kraft paper having a thickness of about 0.0004 inch.

8. A thin kraft paper having a thickness materially less than 0.0005inch, the pores of which are substantially closed, and the fibers ofwhich are relatively fine and frayed out.

9. A substantially non-porous kraft paper having a thickness of about0.0004 inch.

10. In a process for making thin kraft paper the steps which compriseprolonging the beating in the individual stages of adjustment of thebeater roll with respect to the bed plate and controlling the beatingaction in each stage so that the individual fibers can sutiicientlyhydrate to withstand further and more severe heating to ultimatelyproduce fibers frayed out sufficiently to yield on being subjected topaper making steps a paper of the order of 0.000% inch in thickness.

In witness where0f,-I have hereunto set my hand this 23rd day ofJanuary, 1930.

AMOS L. ALLEN.

